The overall goal of this project is to determine the suitability of P-glycoprotein (P-gp) drug binding as a specific target for the development of chemotherapeutic drugs. A radioactive photoactive analog of vinblastine has been used to identify a specific vinblastine binding P-gp in multidrug-resistant cell lines. P-gp vinblastine photolabeling was blocked by a number of indole alkaloids previously shown to increase anticancer drug cytotoxicity and increase drug retained by cancer cells. It was also found that there is a correlation between the level of vinblastine photolabeling of P-gp and the cellular collateral sensitivity of CEM/VLB cells to verapamil up to about 40-fold vinblastine resistance. At higher levels of resistance this relationship was not maintained suggesting that collateral sensitivity of these cells to verapamil may be mediated in part my mechanisms other than P-gp. P-gp has also been identified in HL60/Vinc but not HL60/Adr cell lines by photolabeling with vinblastine or the calcium channel blocker, azidopine. The results suggest that mechanisms of drug resistance in HL60/Adr sublines have features which are distinct from cells containing P-gp and may represent a new model for investigating experimental and clinical drug resistance. Overall, the results suggest a complex relationship between the ability of a compound to modulate multidrug resistance and its ability to compete for binding to P-gp.